Underground utilities are some of the forgotten marvels of civil engineering. While it is typical to bury these structures, it is rare and extremely difficult to perform structural evaluations at a later date. The City Creek Aqueduct that was constructed in 1910 in Salt Lake City, Utah provided a rare opportunity to evaluate how an underground structure had fared after 100 years.
The City Creek Aqueduct was an 84-inch diameter cast-in-place concrete storm water outfall. This pipeline runs along North Temple street and contains constant flows from City Creek, as well as storm water from a large urban area. Despite its age, the pipeline was functioning well and was not scheduled for replacement. The long term viability of the pipeline became an issue, due to the current construction of a light rail transit line along North Temple that would connect the international airport to downtown Salt Lake City. The light rail live loads would not exceed the current highway loading on the pipeline. Yet, the prospect of having to replace an already aged pipeline, that would closely parallel or run under the transit line, created the need for a detailed analysis of the pipeline.
Key to understanding the condition of the existing pipeline was to understand the design and construction practices of 1910. While original plans were not available, historical photos and newspaper articles yielded a great deal of information about the pipeline.
It also showed that while civil engineering technology has changed greatly, the need for civil engineers to deal with the public was just as difficult 100 years ago as it is to today.
As residents complained about the nearly annual flooding that would occur due to the spring snow melt, the plan of an aqueduct became popular. During one of these floods, efforts to further raise temporary embankments were met with criticism by the residents. The water superintendent responded, “Well, you see how much manure there is here. These manure dykes are about a foot and half high and their total length is three-quarters of a block. I don’t think that is going to kill anybody.” Comments like this in the 1908 Salt Lake City Herald only served to spur on the eventual aqueduct. Within several years and bond financing of this and other projects, a bid was let. Davis and Heuser was awarded the contract to construct approximately 3,300 feet of the 84-inch diameter concrete pipeline. The low bid of $48,252, approximately $14.50 per foot, was for 7 inch thick reinforced concrete. An 11 inch thick non-reinforced concrete pipeline was bid as an alternative, but was not found to be as competitive.
The project included a rushed schedule that would require nearly 50 feet of pipeline to be constructed per day. As pre-cast concrete pipeline was not available at the time, the pipeline was cast using an outer frame of round wood staves, with an inner frame of four, removable, quarter-sections. Construction photos indicated that the excavation and pipeline construction was performed with hand laborers and little mechanical help. The newspaper reported that “cave-ins” had occurred due to the rushed construction and “taking away some forms from the concrete before it had time to set”. Yet, the contractor was working at a record pace and finished far before the deadline, constructing as much as 140 feet of pipeline in a day.
Ensign Engineering, of Salt Lake City, Utah was hired by Salt Lake County to perform a condition assessment followed by a structural analysis of the pipeline. The condition assessment showed portions of the pipeline that looked brand new, but other sections had experienced spalling or reinforcing corrosion. Scour, from the constant flow of water, was also seen in some sections.
As the long term viability was a concern regardless of the observed condition, cores were taken to determine the structural capacity of the pipeline. Core samples showed compressive strengths of the concrete ranging from 2,830 psi to 5,750 psi. The core samples showed voids in some samples and larger aggregate than would be allowed in concrete mixes today. The reinforcement was a single cage triangular steel reinforcement that had an area of approximately 0.103 in2/ft (somewhere between a #2 and #3 bar).
While it was difficult to perform a structural analysis as the pipeline conditions varied throughout the length, it can be summarized that both the compressive strength and reinforcement area are less than would be required today. For example, a newly designed concrete pipe of the same diameter would have the same 7 inch wall thickness but steel reinforcing that was nearly four times greater than the original pipeline. Concrete pipe of today has a design strength of 4,000 psi, but 28 day compressive strength breaks often reach 8,000 to 9,000 psi.
Dave Alter, the condition assessment structural engineer from Ensign Engineering noted, “This pipe represents an amazing project for its time, but the pipe’s durability wouldn’t meet today’s standards. The pipe’s stability was mostly based on the soil compaction around the pipe. Based on the performance of the original pipe, concrete pipe with today’s design and quality control should easily last 100 years.”
As part of the light rail project, precast reinforced concrete pipe of the same size has been used to the replace original pipeline. Other sections of the pipeline that are less affected by the light rail line will be left in place and with more frequent condition assessments planned for the future.
- 1,128 feet of 84 inch precast concrete pipe
- 320 feet of 10' x 3' reinforced box culvert